(i) Field of the Invention
This invention relates to a centrifugal type friction clutch which permits a greater freedom in setting variations of torque capacity in power transmission, and a coupling or starter mechanism employing such a centrifugal friction clutch.
(ii) Description of the Prior Art
There have been known and used in the art the so-called lock-up clutches for coupling mechanisms in vehicular automatic transmissions for the purpose of enhancing the efficiency of power transmission. The lock-up clutches are generally classified as either a piston type or a centrifugal type, of which the piston type has drawbacks in that it incurs a high cost and requires a complicated control. With regard to the centrifugal type lock-up clutch, U.S. Pat. No. 4,305,493 discloses a centrifugal type friction clutch using a slidable friction shoe 40 having an engaging portion and end portions folded inwardly in a C-shape, as shown in FIGS. 1 to 3, here in combination with a centrifugal weight 50 held in the folded end portions, a load spring 60 interposed between and abutting against the centrifugal weight 50 and friction shoe 40, and a retractor spring 80 interposed between the load spring 60 and centrifugal weight 50 and abutting, at its ends against, a pair of projections 70 provided on one rotary coupling member.
The prior art centrifugal type friction clutch illustrated in FIGS. 1-3 in the manner as follows.
(a) Los-speed non-operating range (FIG. 1)
The friction shoe 40 is abutted against the paired projections 70 by the reaction force of the retractor spring 80, forming a leading contact portion 40A in driving power transmission and a leading contact portion 40B in driven power transmission, at the opposite ends portions of the friction shoe 40 and load spring 60.
(b) Middle- & low-speed centrifugal clutch range (FIG. 2)
The friction shoe 40, centrifugal weight 50 and retractor spring 80 are urged toward the engaging surface of the opposing rotary coupling member, and the friction shoe 40 and centrifugal weight 50 are separated from each other by centrifugal force.
(c) High-speed operation range (FIG. 3)
As the rotational speed is increased, the centrifugal weight 50 is abutted against the paired projections 70. Consequently, the centrifugal weight 50 will exert no further load acting on the engaging surface of the apposing rotary coupling member.
When the above-described centrifugal friction clutch is used on a motor vehicle, it becomes necessary to raise the operating range of the lock-up clutch above a certain rotational speed to reduce engine torque variations to the point of ensuring comfortable drive of the vehicle. However, in the case of conventional clutches employing centrifugal weights, the clutch torque capacity varies in the fashion of a quadratic curve as shown by the characteristics curve Co of FIG. 16, and becomes smaller at low speeds, broadening the clutch slippage region in the ordinary operating range of the lock-up clutch. This is a great disadvantage from the standpoint of transmission efficiency and durability of the clutch.